Excessive moisture causes damage within many lighting assemblies. For instance, electrical and electronic items may be ruined due to excessive moisture within a lamp enclosure. Examples of lighting assemblies which are subject to undesirable moisture include, for example, automotive headlamp units and other outdoor lighting assemblies where on/off cycling of a lamp within the enclosure results in moisture build-up. As used herein, the term “moisture” is intended to refer to water that is diffused or condensed, whether in liquid form or vapor form, from the ambient atmosphere.
The problem of moisture build up within lamp enclosures is particularly acute in automotive lamps. Modern vehicle head lamps, brake lamps, running lamps, turn signal lamps, fog lamps, back-up lamps and parking lamps (collectively “lamps” or “vehicle lamps”) typically have one or more light bulbs located in an enclosed housing. It is critical to the effective operation of the light to prevent water, dirt, oils and the like from reaching the bulbs, the reflective surfaces, the lens or the housing. However, thermal cycling due to bulb operation, changes in the environment and vehicle operation can cause moisture to condense on the interior of the housing and inhibit light output from the lamp. Moreover, components within a lamp may be damaged by such condensation.
One known means of reducing moisture related problems is to disperse moisture by providing greater airflow across, or through, the enclosure. Thus, vent systems intended to reduce condensation often employ some means of increasing airflow through the lamp housing. However, it can be very difficult to provide sufficient airflow to reduce moisture. Moreover, attempts to increase the vent opening sizes can exacerbate problems such as contamination, etc. In general, the atmospheric air outside of a lamp housing is below the water vapor saturation point, and the air flowing through the housing has the capacity to remove condensation from the lamp housing by removing water vapor from the housing. Vent systems using this means of condensation reduction generally have vent openings in more than one location. The openings are often placed in locations where airflow past the vent opening enhances airflow through the housing. Thus, location of these vent systems can be an important consideration. However, vent systems that provide a means of increasing airflow through the lamp housing often have a negative effect on lamp performance. Specifically, these venting systems often create an opportunity for foreign materials and liquid water to enter the vehicle lighting system.
Another means of reducing moisture in an enclosure is to place a drying agent or desiccant within the enclosure. Desiccants can operate by several fundamental mechanisms including absorption, adsorption, and reaction. Absorption occurs when a substance (e.g., water vapor) penetrates the inner structure of another (the absorbent). Adsorption occurs when a substance (e.g., water vapor) is attracted and held onto the surface of another (the adsorbent). Reaction occurs when the substance (e.g., water vapor) reacts with the desiccant to form a chemical bond with water. As the terms “desiccants” or “drying agents” are used herein, they are intended to refer to any material which absorbs, adsorbs, or reacts with water vapor from the air and is thereby able to reduce the moisture in the air within a lighting enclosure.
Many desiccants will desorb or release adsorbed or absorbed moisture when heated in a process called regeneration. Such desiccants are commonly referred to as regenerating desiccants. In contrast, Non-regenerating desiccants retain adsorbed, absorbed, or reacted moisture when heated.
Regenerating Desiccants have been proposed for use in lamp enclosures. For example, U.S. Pat. No. 6,422,729 to Rohrbach et al. and U.S. Pat. No. 6,709,493 to DeGuiseppi et al., the latter commonly assigned herewith, each suggest using a regenerating desiccant to control moisture in a lamp enclosure. Rohrbach et al. teaches providing a regenerating desiccant within a vent assembly installed in a lamp. When the lamp is turned on, the heat from the bulb causes temperatures inside the housing to rise and the air within the housing to expand. The heating of the air causes its relative humidity to decrease. This hot, expanding, low humidity air is driven out of the housing through the vent and past the desiccant causing the moisture contained within the desiccant to desorb thus regenerate. When the lamp is turned off, ambient air entering the lamp is intended to be dried by contact with the regenerated desiccant. Deguiseppi et al. teaches using a diffusion tube that creates a pathway from the desiccant to the atmosphere.
Under certain conditions, particularly in cold, damp weather, desiccant regeneration can be problematic. Much of the moisture desorbed from the desiccant and the plastic components within the lamp cavity when the lamp is on, may remain in the space within the housing. Moreover, when the bulb is turned off, the air within the housing contracts and air, which may contain additional moisture, is pulled into the housing through the vent. The housing and the air within it may cool relatively rapidly compared to the desiccant itself. Because the desiccant recovers absorptive or adsorptive capacity only as it cools, the moisture that remains distributed within the housing may condense on, for example the cold surface of the lens or housing.
Non regenerating desiccants have been used in, for example, vented automotive headlamps. However, it has proven difficult to provide enough desiccant to meet the design life of most enclosures because significant amounts of moisture are present in the lamp when assembled or enter the headlamp through the venting system by moisture diffusion from the outside environment and during each cycle that the head lamp is energized and de-energized. Depending upon the environment in which the lamp is used, a non regenerating desiccant may become fully saturated after only a few months use in a vented lamp enclosure.